In vehicles in general, various different power train configurations occur, e.g. the gearbox may take the form of a manually operated gearbox or an automatic gearbox. It is often desirable for heavy vehicles to be drivable in as comfortable a way for the driver as possible, which usually entails the gear changes in the gearbox being effected automatically by means of the vehicle's control system. Gearboxes which change gear automatically have therefore also become increasingly usual in heavy vehicles.
This automatic gear changing is often effected in heavy vehicles by using control systems to effect gear changes in “manual” gearboxes (also called AMT, automated manual transmission), partly because they are substantially less expensive to make, but also because of their greater efficiency.
“Manual” gearboxes which change gear automatically are therefore usual in heavy vehicles largely used on motorways/national highways.
This gear changing may be effected in various different ways, one of which involves using a clutch automatically operated by the vehicle's control system for up/downshifts, in which case the driver only needs access to an accelerator pedal and a brake pedal.
In principle, the clutch need only be used to set the vehicle in motion from stationary, as other gear changes can be effected by the vehicle's control system without using the clutch at all, since they are instead effected “torque-free”. It is also possible for the automatically operated clutch to be used for only certain gearchange steps or only in certain upshifts or downshifts.
For comfort reasons, however, the automatically operated clutch is in many cases used in all or substantially all upshifts and downshifts.
The efficiency of automatic gearboxes of the type often fitted in cars is far too low to be justified other than in, for example, urban buses and distribution vehicles in towns where the vehicles usually have frequently to come to a halt and then move off again.
It is however becoming increasingly common for even vehicles such as urban buses and distribution vehicles in towns to be provided with manual gearboxes operated by control systems instead of a conventional automatic gearbox.
Control of the automatically operated clutch is conducted by using the vehicle's control system to operate a clutch actuator which may for example consist of one or more pneumatically operated pistons which act upon a lever arm whereby the clutch is opened/closed by using said pistons to effect a lever arm movement. The clutch actuator may also be of an electrical type.
A manual clutch is normally activated less than the amount of use to which clutches/clutch actuators are subject during automatic operation by means of the vehicle's control system.
The vehicle's control system may for example allow slipping of the clutch over relatively long amounts of time, e.g. to achieve as comfortable a gear change as possible, or when moving off from stationary in a relatively high initial gear.